In recent years, from the viewpoint of reduction of load applied on air conditioning equipment based on high concern for energy conservation measures, demands are increasing for near-infrared reflective films configured to be pasted on window glasses of buildings and vehicles so as to cut off penetration of heat rays of sunlight.
As methods of forming such a near-infrared reflective film, dry film-formation methods such as vapor deposition and sputtering are proposed so as to for a lamination film in which a high refractive index layer and a low refractive index layer are laminated alternately. However, the dry film-formation methods have disadvantages such as need to use a large-sized vacuum apparatus for film-formation, high production cost, difficulty in formation of a film with large area, and restriction on substrates limited to heat-resistant material.
It has been known to manufacture near-infrared reflective films by wet film-formation methods in place of the dry film-formation methods having the above advantages.
For example, Patent Document 1 discloses a method of preparing a high refractive index layer coating liquid by dispersing a heat hardenable silicone resin or ultraviolet hardenable acrylate resin each including metal oxides and metal compound particle in an organic solvent, and coating the high refractive index layer coating liquid onto a substrate so as to form a transparent lamination film by a wet type coating method employing a bar coater. Further, Patent Document 2 discloses a method of preparing a high refractive index layer coating liquid containing a rutile type titanium oxide, a heterocyclic system nitrogen compound (for example, pyridine), an ultraviolet hardenable binder, and an organic solvent, and coating the high refractive index layer coating liquid onto a substrate so as to form a transparent lamination film by a wet type coating method employing a bar coater.
In addition, Patent Document 3 discloses a method of laminating layers alternately by using a methanol dispersion slurry of spherical rutile type titanium dioxide grains and a methanol silica sol.
However, in the methods disclosed by Patent Document 1 and Patent Document 2, since an organic solvent is mainly used as the medium of the high refractive index layer coating liquid, a volume of the organic solvent is scattered, which causes environmental problems. Further, in the methods disclosed above, an ultraviolet hardenable binder and a heat hardenable binder is used as a binder, cured by ultraviolet or heat after formation of a high refractive index layer. Accordingly, the resultant film has physical properties which lack flexibility.
Furthermore, since the slurry in which rutile type titanium dioxide grains are dispersed in an organic solvent with a surface treating agent is used, there are problems of wide particle size distribution, uneven refractive index in a plane of a coated film, and color change of a coated film with time due to the influence of the surface treating agent.
Moreover, in the method described in Patent Document 3, since the film formation is achieved by binding among particles, the resultant film is brittle. In addition, in the high refractive index layer formed by binding among the rutile type titanium dioxide grains, there are problems that haze becomes high due to voids caused on the boundary surfaces of particles.